Microwaves propagation through a small fire curtain at 3.55 GHz and 6.26 GHz and the impact on vital signs detection by using a four-element patch antenna array

The impact of the presence of a fire curtain not wider than 7 cm on an air path 150 cm long of two continuous waves at frequencies of 3.55 GHz and 6.26 GHz was quantified. Two identical patch array antennas with four linear elements were used at emission and reception and they were designed and characterized specifically for this purpose. The magnitude and phase of the transmission coefficient were measured by a vector network analyzer in all the experimental sessions. Comparing the air path and the air-plus-fire path of the waves, it resulted that fire induced shifts in magnitude and phase that depended on frequency and on the experimental setup. The heating of the antenna could have contributed to the overall observed effects. The detection of the human heartbeat by Doppler radar technique was not much influenced by the presence of fire, but the amplitude of the detected beats was increased. The array antennas produced noisy signals for the vital sign detection and the origin of this behaviour was hypothesized.